Income Taxes (Details) - USD ($) $ in Millions	1 Months Ended		3 Months Ended
	Apr. 30, 2018	Mar. 31, 2018	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Income Tax Contingency [Line Items]
Effective tax rate			10.80%	32.10%
Income benefit			$ (113)	$ (315)
Unrecognized tax benefits		$ 0	$ 0
Significantly increase (decrease) in amount of unrecognized tax benefits			12 months
ALABAMA POWER CO
Income Tax Contingency [Line Items]
Income benefit			$ (82)	(126)
GEORGIA POWER CO
Income Tax Contingency [Line Items]
Income benefit			(93)	(156)
GULF POWER CO
Income Tax Contingency [Line Items]
Income benefit			$ (13)	$ (14)
MISSISSIPPI POWER CO
Income Tax Contingency [Line Items]
Effective tax rate			(34.70%)	(58.70%)
Income benefit			$ 4	$ 27
SOUTHERN POWER CO
Income Tax Contingency [Line Items]
Effective tax rate			(647.00%)	(385.90%)
Income benefit		50	$ 99	$ 52
SOUTHERN Co GAS
Income Tax Contingency [Line Items]
Income benefit			(104)	$ (150)
Investment Tax Credit Carryforward
Income Tax Contingency [Line Items]
Tax credit carryforward, amount		$ 2,300	$ 2,300		$ 2,100
Subsequent Event | SOUTHERN POWER CO
Income Tax Contingency [Line Items]
Income benefit	$ 4